Organisms have evolved mechanisms for controlling the acidity of their intracellular and extracellular fluid compartments to within narrow physiological limits. On the systemic level, cells that function as hydrogen ion sensors in the brain, the heart, the gut,and the tongue evoke reflexive responses that serve to mitigate and ultimately eliminate the acidic challenge. Sour taste receptors detect a decrease in intracellular pH, but in the case of strong acids such as gastric acid (HCI) the mechanism by which protons enter taste cells remains unknown. Preliminary data from NADPH oxidase knockout mice indicate that protons cross the apical membranes of taste cells through two hydrogen ion channels. 1) About 60% of the taste neural response to HCI is absent in NADPH oxidase knockout mice relative to controls, suggesting that one taste proton channel is linked to NADPH oxidase activity. 2) The remaining response to HCI in NADPH oxidase knockout mice is enhanced by cyclic AMP. We will investigate the function of both channels in detail by recording from taste nerves in anesthetized wild type and NADPH oxidase knockout mice under voltage clamp conditions, and by making hydrogen ion flux measurements across the apical and basolateral membranes of taste cells using a polarized single taste bud preparation with fluorescence imaging methods. Transdaction for the phasic part of the sour response is calcium-independent while the tonic part depends on calcium. New data indicate that the phasic transduction involves a decrease in taste cell volume induced by a decrease in intracellular pH. The pH-induced volume decrease depends on cytoskeletal actin. Chemical disruption of the cytoskeleton or osmotically pre-shrinking the cells blocks the phasic response, but not the tonic. We shall explore the link between the phasic sour response and cell volume changes in detail. pH recovery mechanisms in taste cells shape the sour taste response. We have evidence that one of these is pendrin, a chloride-bicarbonate exchanger. We will investigate its role and the role of related transporters in sour responses to carbon dioxide. Since pendrin also transports formate, we will investigate its role in formic acid taste responses. The proposal investigates the three key processes in sour taste: 1) acid entry mechanisms, 2) separate phasic and tonic transduction mechanisms, and 3) pH recovery and adaptation mechanisms. The research will reveal how the sour taste defense mechanism functions to prevent the ingestion of acids.